Zinc oxide film and method for making

ABSTRACT

A method for depositing a solid film of ZnO onto a substrate from a reagent solution includes a reservoir of reagent solution maintained at a sufficiently low temperature to inhibit homogeneous reactions within the reagent solution. The reagent solution contains a source of Zn, a source of 0, and multiple ligands to further control solution stability and shelf life. The chilled solution is dispensed through a showerhead onto a substrate. The substrate is positioned in a holder that has a raised structure peripheral to the substrate to retain or impound a controlled volume (or depth) of reagent solution over the exposed surface of the substrate. The reagent solution is periodically or continuously replenished from the showerhead so that only the part of the solution directly adjacent to the substrate is heated. A heater is disposed beneath the substrate and maintains the substrate at an elevated temperature at which the deposition of a desired solid phase from the reagent solution may be initiated. The showerhead may also dispense excess chilled reagent solution to cool various components within the apparatus and minimize nucleation of solids in areas other than on the substrate. The deposited film may be annealed after deposition and may be doped to enhance selected characteristics. The ZnO films made by the process have distinctive electrical and optical properties and are suitable for a variety of electronic and optical devices.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. patent application Ser. No.12/151,465 filed by the present inventor on May 7, 2008, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a system and method for chemically coating avariety of surfaces with zinc oxide thin films for various applicationsincluding electronics, and to zinc oxide films having novel properties.

2. Description of Related Art

Numerous coating processes are commonly employed in industrialapplications, including electroless chemical, chemical vapor andphysical vapor depositions. Physical vapor deposition is commonly usedin semiconductor manufacturing applications, often employing expensivevacuum techniques in order to sustain a relatively high film growthrate. Many such processes, while performed at high temperatures (e.g.,greater than 300° C.), are non-equilibrium, often resulting innon-stoichiometric proportions. Also, due to the nature of thedeposition processes, the deposited films often include relatively highdefect densities. In the case of semiconducting devices, such highdefect levels can limit electrical performance characteristics. Insemiconductor device fabrication wherein p-n junctions are formed in apartial vaccuum by depositing one film over a second film or a substrateof different conductivity type, the conventional evaporative andsputtering techniques may provide unsatisfactory film qualities. As analternative, relatively more expensive techniques such as Chemical VaporDeposition (CVD), Molecular Beam Epitaxy (MBE), pulsed laser deposition,and atomic layer epitaxy, are useful, especially with formation of III-Vcompound semiconductor materials, but satisfactory deposition processeshave not been available for fabrication of thin film II-VI compoundsemiconductor materials.

Chemical bath deposition (CBD) is a low cost, low temperature techniquewhich, under certain limited conditions, can provide high quality thinfilm growth of desired stoichiometry. However, the technique hasprovided low growth rates, e.g., 20 to 30 Å/minute, and the grown filmthickness per bath cycle is limited to around 1000 Å rendering itnon-suitable for volume manufacture. Generally, for products, such assolar diodes and light emitting diodes, film thicknesses on the order of3,000 to 20,000 Å are needed. With traditional chemical bath depositiontechniques, efforts to increase film growth rates typically result indegradation of film quality. Thicker films of higher quality, however,can be attained by cyclic deposition of stacked relatively thin layers.This, of course, is time consuming and generally can be an expensiveendeavor, wherein relatively expensive chemicals are wasted due todeposition of film materials on unwanted surfaces such as the equipment.

Conventional CBD is carried out with the source solution held atslightly elevated temperature, typically ˜85° C. [see, for example,Oladeji and Chow, “Optimization of Chemical Bath Deposited CadmiumSulfide Thin Films,” J. Electrochem. Soc. 144 (7): 2342-46 (1997)]. Inthis situation, heterogeneous nucleation of the CdS film on thesubstrate must compete with homogeneous nucleation of colloidal CdSparticles within the stirred reactant solution. Particulates representnot only a waste of reagents but also a source of defects in thedeposited film.

A process for depositing ultra-thin semiconductors is taught byMcCandless et al. in U.S. Pat. No. 6,537,845. The process uses apremixed liquid containing Group IIB and VIA ionic species and acomplexing agent. The solution is applied to a substrate heated to atemperature from 55 to 95° C., forming an ultra-thin (100-500 Å)coating. For thicker coatings, the process can be repeated. The processtaught in '845 suffers from several noteworthy shortcomings. First,using a single complexing agent (generally taught to be NH₄OH) preventsadequate process control: at a low concentration the solution is sounstable that unwanted homogeneous nucleation can occur, whereas at highconcentration the activation energy required to form the film becomes sohigh that the claimed substrate temperature may not be able to overcomeit to cause a film growth. Second, the thickness of film that can begrown in a single step is very small, so to grow a film of 0.1 μm orgreater, multiple cycles are needed and this will tend to introducegreater concentrations of defects. This will also make the processcumbersome and less manufacturing friendly.

Films have also been prepared using the flowing liquid film process astaught by Ito et al. [Preparation of ZnO thin films using the flowingliquid film method, Thin Solid Films 286: 35-6 (1996)]. The depositionprocess involved the reaction of zinc chloride and urea at 70° C.according to the reaction:

ZnCl₂+NH₂CONH₂+2H₂O→ZnO+2NH₄Cl+CO₂.

The use of NH₂CONH₂ as a single ligand will result in excessivehomogenous reaction, and ZnO film growth takes place mostly byparticulate adsorption. ZnO so formed is less transparent and of littleor no practical use. Furthermore, the side of the substrate next to theincoming solution gets the full dose of the solution, hence high growthrate, whereas the side of the substrate at solution exit receives theleast dose, hence low growth rate. As a result, the film non-uniformitywill increase with increasing substrate size. Since all the flowingsolution sees the heat provided by the substrate, film growth will takeplace on the substrate plus any part of the system on contact with thehot growth solution, leading to material waste. If the particulategeneration happens in the enclosed chamber, these could be trapped andget adsorbed unto the substrate, leading to poor quality film.

In U.S. Pat. App. Pub. 2003/0181040 as taught by Ivanov et al., filmshave also been prepared on semiconductor substrates using a sealedchamber filled with growth solution maintained at high pressure about 2atmosphere and temperature at 0 to 25% below the boiling point of thesolution. The use of high pressure is needed to keep the rigid substratein place, and the cost associated with this is not trivial. Therelatively large volume of solution in the chamber is heated completelyby the heater located outside the chamber or in the substrate holder orboth. This arrangement forces the hot growth solution to be in contactwith the substrate and substantial part of the chamber system, andresults in wasteful film growth on unwanted areas. The latter will alsoincrease the cost of keeping the chamber clean to prevent particle buildup. Subjecting a relatively large volume of growth solution in chamberat high pressure and temperature will not only encourage the desiredheterogeneous reaction responsible for film growth but alsosubstantially increase the unwanted homogenous reaction; the latter willlead to fast depletion and waste of material, and this will in turnincrease the production cost.

These facts were recognized by Ivanov, who through U.S. Pat. No.7,235,483, attempted to minimize the material waste, by having thesubstrate heated and cooled instantaneously, and using the hightemperature only during the growth regime when it is needed, especiallyduring the bulk film growth step. For Cu interconnect, the focus of thatwork, where 500 Å or less Cu metal seed and CoWP Cu capping layer areneeded, the bulk deposition step may be short enough to prevent quickdepletion of chemicals. However, for semiconductor film depositionneeded for solar cells and other optoelectronics applications where therequired film thickness is more than 1000 Å, the high temperature bulkdeposition step time will be longer and the instantaneous heating andcooling will not help. Generally, in electroless semiconductor filmdeposition catalyzed substrate surface is not required. One wouldtherefore expect the growth rate on the substrate as well as other partsof the chamber system in contact with the hot solution to be about same;the consequence of this is substantial material waste. This will makethe chamber cleanliness even a much bigger challenge. The operating costand waste management in using this system for semiconductor filmmanufacturing will be prohibitive. It should also be noted that thisapproach is only applicable to rigid substrates; flexible substratescannot be used.

The growth of ZnO films, in particular, is of great technologicalinterest and a number of techniques have been reported. Methods includethe following: radical-source molecular beam epitaxy [see R. Hunger etal., Mat. Res. Soc. Symp. Proc. Vol. 668 (2001)]; pulsed laserdeposition [see T. Oshima et al., Thin Solid Films 435: 49-55 (2003);also Y Ryu et al., Appl. Phys. Letters 83[1]: 7 (2003)]; metalorganicvapor phase epitaxy [see Y. Ma et al., Journal of Crystal Growth 255:303-07 (2003)]; and DC reactive sputtering [see A. Mosbah et al.,Surface and Coating Technology 200: 293-96 (2005)]. In general, filmsdeposited by these techniques exhibit carrier concentrations rangingfrom about 5×10¹⁵ to 2×10¹⁸ and resistivities ranging from 0.3 to 565Ω-cm.

Objects and Advantages

Objects of the invention include the following: providing a bathdeposition apparatus capable of depositing ZnO on a substrate whileminimizing homogeneous nucleation of the same or similar phase withinthe bulk of the fluid bath; providing a bath deposition apparatuscapable of depositing ZnO films having improved physical, chemical,optical, or electrical properties; providing a ZnO bath depositionprocess that is more easily controlled; providing a ZnO bath depositionprocess that uses reagents more efficiently; providing a ZnO bathdeposition process that allows localized heating of a relatively smallvolume of the bath while maintaining the remainder of the bath at arelatively lower temperature; providing a ZnO film having improvedphysical, electrical, and optical properties; and, providing a ZnO filmdeposited on a selected substrate whereby the film and substrate mayform part of an electrical or optical device. These and other objectswill become apparent on reading the specification in conjunction withthe accompanying drawings

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method for depositinga solid ZnO film onto a substrate from a reagent solution comprises thesteps of:

-   a. providing a supply of reagent solution containing a source of Zn    and at least two ligands maintained at a first temperature at which    homogeneous reactions are substantially inhibited within said    reagent solution;-   b. dispensing a controlled flow of the reagent solution from a    showerhead assembly;-   c. positioning the substrate to receive at least a portion of the    controlled flow of reagent over a selected area of the substrate;-   d. providing a raised structure peripheral to the selected area    whereby a controlled volume of reagent solution may be maintained    upon the substrate and replenished at a selected rate; and,-   e. heating the substrate and the controlled volume of reagent    solution upon the substrate to a second temperature, higher than the    first temperature, whereby deposition of zinc oxide from the reagent    solution may be initiated.

In accordance with another aspect of the invention, a multilayerstructure comprises: a substrate material; and, a thin film of ZnOdeposited upon the substrate, the ZnO film having a carrierconcentration less than about 10¹⁵ cm⁻³ and a resistivity greater thanabout 600 Ω-cm.

BRIEF DESCRIPTION OF THE DRAWINGS

A clear conception of the advantages and features of the invention, andof the components and operation of exemplary systems provided with theinvention, will become more readily apparent by referring to thedrawings accompanying and forming part of this specification, in whichlike numerals designate like elements in several views. The features arenot necessarily drawn to scale.

FIG. 1 illustrates schematically the general flow of reagents inaccordance with one aspect of the invention.

FIG. 2 illustrates typical process inputs and outputs for a systemcontroller suitable for use with the present invention.

FIG. 3 illustrates schematically a plan view of a substrate holder andCBD chamber in accordance with one aspect of the present invention.

FIG. 4 illustrates schematically a vertical section along A-A′ of thesubstrate holder and CBD chamber illustrated in FIG. 3 in accordancewith one aspect of the present invention.

FIG. 5 illustrates schematically a vertical section of the substrateholder and CBD chamber in accordance with another aspect of the presentinvention.

FIG. 6 presents a process flow diagram for a two-layer film stackprepared according to one aspect of the invention.

FIG. 7 illustrates schematically a number of deposition chambers inaccordance with the present invention arranged in a cluster toolconfiguration.

FIG. 8 illustrates schematically another way in which multipleprocessing chambers may be arranged.

FIG. 9 illustrates schematically a plan view (upper) and cross-sectionalview (lower) of an embodiment of the invention adapted for depositiononto a continuous flexible substrate.

FIG. 10 compares the transmission spectra of a ZnO film grown by theinventive method, as grown and after annealing at 400° C.

FIG. 11 shows the X-ray diffraction pattern of one example of theinventive ZnO film after annealing at 400° C.

DETAILED DESCRIPTION OF THE INVENTION

There has been a need for a chemical system that can create high qualityfilms, e.g., semiconductor films, at high formation rates, whileproviding a relatively uniform film thickness across the entiresubstrate surface over which the film is formed. In accordance withseveral embodiments of the invention, such a high quality semiconductorfilm is formed by a replacement reaction wherein the system provides acontinuous or replenishable supply of chemical processing solution. Inseveral examples, the solution reacts about the surface of a heatedsubstrate. The substrate temperature may be controlled to exhibit asubstantially uniform temperature across the surface. That is,temperature differentials along the surface over which the film isformed are limited in order to effect a substantially constant reactionrate along the surface, thereby assuring a relatively uniform filmgrowth rate. Further, the pH and composition of the solution may becontinuously monitored and maintained to improve the stability of theprocess and, hence, the quality of the deposited film, e.g.,stoichiometry, defect density, uniformity and consistency of dopantdistribution. The system may be operated above atmospheric pressure toincrease the rate of film growth.

FIGS. 1-4 illustrate simplified schematic diagrams of a Film GrowthSystem (FGS). In some of the examples that follow, the illustrated filmscomprise II-VI compound materials but the system is useful for a widevariety of film compositions and numerous other embodiments arecontemplated in order to effect film growth according to the principlesdescribed herein. The system of FIGS. 1-4 and other embodiments areshown with specific geometries but these are merely exemplary. Forexample, the shapes of substrates, chambers, chamber interiors, showerheads and numerous other features described herein may be readilymodified without departing from the spirit and scope of the invention.It is also to be understood that, throughout the written description andthe figures, numerous like components common to different embodimentsare designated with similar names and, in such cases, descriptions andfunctions described for a component with reference to one or moreembodiments are applicable to similarly named components of otherembodiments.

As used herein the term film forming surface means the surface of aworkpiece, e.g., a substrate, over which a desired film is grown. Asdescribed below, the film forming surface may be the surface of asubstrate that faces a showerhead in order to receive a direct flow orspray of chemical processing solution as illustrated for severalembodiments herein. Further, the illustrated embodiments are describedwith reference to an exemplary orientation wherein the reaction chamberreceives the workpiece or substrate in a mounting such that theworkpiece is in a level position with respect to a major surface, e.g.,during the film growth process the surface over which a film is grown islevel. The illustrated chambers are also depicted as having bases,showerheads and lids which are positioned in a level orientation duringfilm growth processes. However, other orientations of the substrate andvarious components may be preferred for other embodiments andapplications without departing from the spirit and scope of theinvention.

The FGS of FIG. 1 includes a reaction chamber 10 and a chilled solutionreservoir 11 for retaining and chilling chemical processing solutionwhich may be held at a stable, low temperature preferably less thanabout 25° C. and more preferably in the range of 10 to 25° C. or lower.At the preferred temperatures, homogeneous reactions are substantiallyinhibited and the solution has adequate shelf life and minimalhomogeneous precipitation of particulates. The FGS further includes asolution supply - subsystem for moving the chemical processing solution(while maintained at a low temperature) from the reservoir to thechamber, and a solution return subsystem for cycling partially spentsolution from the chamber back to the solution reservoir. The solutionsupply subsystem includes a supply line filter F2 for removingparticulates from the solution, a supply line pump P2 for supplying thesolution to the reaction chamber, and a liquid control panel 12 underthe direction of a system controller. The liquid control panel 12includes a series of conventional valves and flow controllers forselectably injecting and varying flows of liquids into the processingchamber 10. For example, the liquid control panel 12 may deliver thechemical processing solution and one or more chemicals for chambercleaning or for cleaning of the surface of the substrate over which adesired film is grown.

Chemical processing solution from the reservoir is supplied to theliquid control panel by the supply line pump after passing through thesupply line filter F2. The solution return subsystem includes aplurality of valves for controlling flows of various liquids from theprocessing chamber, a return line reservoir 13, a return line filter F1for removing particulates from the returning solution, and a return linepump P1 for sending the filtered solution to the process solutionreservoir 11. Proper control and regulation of the liquid flowingthrough the liquid control panel is effected by the flow controllersunder the direction of the system controller. Solution reservoir 11 mayoptionally include such familiar components as high and low levelswitches 14, 15, temperature sensor 16, pH meter 17, and chemicalanalyzer 18.

The reaction chamber 10 illustrated schematically in FIGS. 3-4 includesa substrate holder assembly 31 with clamps 32 arranged to securesubstrate 33 under spacer 35, a delivery system that includes ashowerhead 41 for supplying and distributing processing solution withinthe chamber and over a substrate (workpiece) 33. The substrate holderassembly 31 comprises an insulative housing 43 with a heater block 44formed therein. The combination of the heater block and an upperinsulative surface (over which a substrate is placed) is referred toherein as a platen. Various of these components are illustrated ashaving circular or rectangular shapes, but the functionality of thereaction chamber is not limited to these example geometries. Acontinuous spacer 35 may be positioned about the upper, exposed surfaceof the workpiece, e.g., along the periphery thereof or beyond theperiphery. In other embodiments the continuous spacer may be replaced bya spacer including apertures or slots or may be replaced by severalindividual spaced-apart spacers wherein the separation facilitates exitflow of processing solution from the reaction region adjacent to theexposed upper substrate surface through the solution return subsystem tothe chilled reservoir 11. The chamber may further include a pressuregauge, a gas inlet 42, a gas outlet 42′, and a drain line 34 which ispart of the return subsystem that directs partially spent processingsolution from the chamber 10 to the reservoir 11. In this example, tominimize maintenance and extend useful life of the chamber, all chamberparts that are exposed to the chemical solution are preferably made ofchemically inert material, such as polytetrafluoroethylene (PTFE) orperfluoroalkoxy (PFA), or the surfaces of such parts may be coated witha PTFE or PFA film. For example, the spacer 35 (positioned about thesubstrate to create a shallow catch region or substrate opening 49 forretaining flowing portions of the processing solution over the substratesurface) may be formed of a relatively rigid material such as PTFE, PFAor PFA-coated or PTFE-coated metal. The illustrated continuous spacer 35encloses an area of the substrate surface that may be continuously,continually or periodically replenished with processing solution, assolution previously supplied from an overlying showerhead overflows orpours outward from the volume enclosed by the spacer 35 and passes intothe drain line 34.

It will be appreciated that the operation of spacer 35 differsfundamentally from the “containment frame” of McCandless et al. '845 inthat McCandless contemplates a substantially static containment of afixed volume of solution, whereas the present invention relies oncontinuous or periodic replenishment using chilled solution from theshowerhead. Applicant uses this novel feature to further cool adjacenthardware components as well as cool the solution in areas other than theimmediate deposition layer in order to more effectively suppresshomogeneous nucleation.

To prevent leakage of various process liquids or gases, seals or O-rings48 may be placed at various locations where two components face oneanother, as is well known in the art.

Other flow means are contemplated, including provision of flow linesabout the spacer(s) to actively or passively remove partially spentchemical solution from regions overlying the substrate. For reactionsperformed under atmospheric conditions, processing solution overflowingfrom the spacer volume above the substrate is collected at the bottom ofthe chamber and continuously drained. The processing chamber may befilled or continuously purged with filtered air or nitrogen forcontrolling air-born particles during deposition. To effect this, a gassupply valve may be positioned in the gas supply line 42 to controlinjection of the air or inert gas into the chamber while an exhaustvalve is positioned in the gas exhaust line 42′.

A feature of the FIG. 1 embodiment is that the rate of film formation,e.g., the rate of growing II-VI semiconductor materials, in the FGS isrelatively high while the ratio of the volume of processing solutionabout the film formation surface on the substrate to the area of thefilm formation surface over which the film is formed is small, i.e.,small relative to the corresponding ratio present in a conventional,traditional chemical bath. The foregoing statement is made with theunderstanding that the geometry and orientation of the substrate withina traditional chemical bath is typically different than that of the FGSsince the traditional bath typically immerses the substrate in a heatedvessel containing processing solution while the FGS positions and heatsthe substrate to receive a supply of processing solution, e.g., via acontinuous or continual or periodic flow such as from above. Thus adirect comparison of such ratios is not easily ascertained. Nonetheless,according to the invention, the applicable volume-to-surface area ratiois effectively substantially smaller than that which would be present ina traditional bath, e.g., when the substrate is positioned in ahorizontal or vertical orientation inside a vessel filled withprocessing solution or otherwise oriented with respect to a levelsolution surface. For operation in the FGS, the ratio of the volume ofprocessing solution about the film formation surface of the substrate tothe area of the substrate surface over which the film is formed isreferred to herein as the Volume-to-Surface Area Ratio (VSAR). When theVSAR is high, a relatively small fraction of the processing solution iseffectively contributing to the film formation while the remainingportion may result in formation of colloids in the bulk of the solution.In the FGS the spacer allows a limited volume of processing solution tobe positioned above the substrate surface. The spacer may appear like ashallow tank, but actually serves as a temporary, periodicallyreplenished collection region, as portions of the processing solutionremain over the substrate surface for brief periods in order to effectchemical reactions along the exposed surface. The exemplary continuousspacer is secured along the periphery of the substrate with amulti-clamp design (shown in FIGS. 3 and 4) which allows the solution toflow over the spacer. Thus, the volume of chemical processing solutionretained within the spacer confines is generally determined by theheight of the spacer above the substrate, ranging, for example, between0.1 and 10 mm. Depending on the specific process, setting of processparameters (e.g., substrate temperature, flow rate of processingsolution, concentration of reactants in the solution) a spacer with asuitable or optimum height may be chosen. Due to the complexity ofmultiple variables, and the desire to maximize the rate of filmformation, it may be necessary to experimentally determine processgeometries and settings. The FGS may be programmed to deliver acontinuous or periodic flow of processing solution through the showerhead, or may be programmed to deliver metered volumes, to provide auniform delivery of the solution over the film formation surface. With acontinuous spacer formed over a uniformly heated substrate and acontinual or periodic replenishment of processing solution over thereaction region of the substrate, a low VSAR effects a relativelyuniform film growth over the entire region of film growth at arelatively high growth rate suitable for volume manufacture. Among theseveral embodiments disclosed herein, exemplary VSARs range from 0.1 to10 mm, with exemplary corresponding film growth rates ranging from 100to 1000 Å/minute and film thicknesses of satisfactory uniformities(e.g., less than 10 percent) may exceed several microns.

Another feature of the invention is that during the film formationprocess the film growth surface may be kept at a relatively hightemperature while other surfaces within the reaction chamber arerelatively cool. This feature can minimize or prevent formation of filmon surfaces other than that of the substrate. By way of example, thetemperature differential between the growth surface and other surfacesin the reaction chamber can range from 60° C. or 70° C. to 200° C. andmay, for example be 140° C. The chamber pressure, the solution flowrate, and the substrate temperature may be adjusted to achievedeposition rates on the order of 500 Å/min to produce a relativelyuniform film having a thickness on the order of one to five microns. Theachievable uniformity (i.e., measurable based on variation in filmthickness) over a film growth area of 21 cm² is generally less than 10%and in some instances less than 5%. As shown generally in FIG. 1, duringoperation of the FGS the processing solution passes from within thevolume defined by the spacer 35 to the solution return line and theassociated reservoir 13 in order to be pumped back into the solutionreservoir 11 for chilling and recirculation until such time that therequisite chemicals are so spent that the solution should be replaced.In the process of passing into the return line, the processing solutionleaving the volume defined by the spacer may join other portions ofprocessing solution which are relatively cool. That is, such otherportions of processing solution are injected through the showerhead andinto regions of the chamber outside the region of the heated film growthsurface such that these portions are relatively cool. The latter,relatively cool solution may mix with portions of the solution flowingfrom the spacer volume to facilitate temperature reduction of the heatedsolution, thereby limiting formation of undesirable products in thereaction chamber. The solution reservoir may include a temperaturesensor 16, a chemical analyzer 18, a pH meter 17, and a pair of liquidlevel switches 14, 15. The temperature of the solution reservoir iscontrolled by the chiller and the reservoir is typically maintained at atemperature in the range of 10-25° C. A chemical analyzer 18 isconnected to the solution reservoir to monitor the chemical compositionof the reservoir constantly or periodically. When needed, the pre-mixedprocessing solution is added to the solution reservoir from a tankcontaining pre-mixed solution of desired concentration, although thesolution may be stored in a more concentrated form and diluted upon orprior to entry into the reservoir. For example, the system controllermay operate valves associated with the liquid control panel to dispensedistilled (DI) water and precursor solutions directly into the reservoirfor mixing in the solution reservoir. In some chemical processes,deposition rates are significantly influenced by the pH of the solution.The pH meter 17 monitors the pH of the solution bath and the pH may beadjusted under direction of the controller. Liquid level in thereservoir is controllable with first and second liquid level sensorswitches. The first liquid level sensor switch 15 provides a signal tothe controller when a minimum desired level of solution resides in thereservoir. The second liquid level sensor switch 14 provides a signal tothe controller 20 when a maximum level of liquid resides in thereservoir.

The entire system may operate under direction of the system controller,which actuates numerous valves and switches in response to sensorinformation, e.g., signals provided by level switches. The systemcontroller controls delivery of processing solution to the reactionchamber via the solution flowmeter and may also selectively rotate theshowerhead during the chemical process. A thermocouple is provided tocontrol the platen temperature during operation. The controller alsocontrols delivery of pre-mixed processing solution to the solutionreservoir to maintain suitable level of processing solution in thereservoir.

For periodic maintenance of the processing chamber, a chemical solutionfor chamber cleaning may be supplied from a cleaning chemical tank via acleaning chemical supply line. DI water for rinsing the chamber afterchamber cleaning may be supplied from a DI water tank via a DI watersupply line. Delivery of cleaning chemical is controlled by a cleaningchemical supply line valve. Delivery of DI water is controlled by a DIwater supply line valve. A separate drain under control of a dedicatedvalve may be provided to collect the chamber cleaning chemical and rinsewater. Generally, the flow of drain liquid from the chamber iscontrolled by a combination of the solution return line valve and acleaning chemical return line valve under direction of the controller.The collected cleaning chemical may be sent to a chemical recoveryprocessing unit for recovery of chemical ingredients.

In the subject chemical film growth processes, the formation rates maybe increased as a function of pressure. It will be appreciated thatchamber 10 may be maintained at elevated pressure using filtered air ornitrogen injected into the chamber via the gas inlet 42. Chamberpressure may continuously monitored by the pressure gauge 19.

FIG. 3 provides a simplified plan view of the chambers shown in FIG. 1.The exemplary chamber 10 comprises a circular housing and a squaresubstrate holder 31 for processing a square substrate 33. In otherembodiments, the chamber may comprise a circular substrate holder and acircular chamber housing for processing a circular substrate. In stillother embodiments, a chamber may comprise a square substrate holder in asquare chamber housing. The illustrated substrate holder assemblyincludes a clamping unit having a plurality of clamps 32 coupled to acontinuous spacer 35. The exemplary clamping structure shown in FIG. 3includes four clamps. Each clamp is connected to the chamber bottom orbase region by a substrate holder screw (shown in FIG. 4). In anautomated system, the clamping unit may be operated by one or moreactuator mechanisms for automatic clamping of the spacer onto thesubstrate and releasing of the spacer from the substrate. The actuatorsmay be of any familiar type such as hydraulic, pneumatic, orelectromechanical devices. The substrate holder assembly furtherincludes a plurality of substrate holder screws which are connected tothe chamber bottom wall (shown in FIG. 4) for securing the substrateholder assembly. A drain 34 is provided at the chamber base forrecirculation of processing solution to the solution reservoir anddisposal of cleaning chemical and rinse water during periodic chambercleaning.

FIG. 4 depicts a cross-sectional view of the FGS reaction chamber takenalong line A-A′ of FIG. 3. The chamber includes a lower chamber wallhaving a top surface, a showerhead plate 41 having an upper surface anda lower surface, a chamber lid having an upper surface and a lowersurface configured for placement against the top surface of the lowerchamber wall to effect a sealing arrangement during operation of thechamber, and a substrate holder assembly for holding a substrate duringdeposition. The showerhead plate 41, having a plurality of openings in acentral region thereof is positioned along a lower surface of thechamber lid so that it is positioned at a level coincident with the topsurface of the lower chamber wall during chamber operations. Theshowerhead may be secured by a showerhead clamping unit comprisingshowerhead plate nuts and shower head plate screws or may be securedwith a quick release clamping mechanism for an automated operation. Ashowerhead O-ring may be placed between the lower surface of theshowerhead and the top surface of the lower chamber wall to facilitatecreation of an air-tight seal when the chamber is closed with the lid.The chamber lid is placed over the showerhead plate and secured by achamber lid clamping unit comprising an upper chamber lid nut, a lowerchamber lid nut and a chamber lid screw. The lower chamber wall includesa gas inlet 42, a gas outlet 42′, and a processing solution drain 34.

Depending on the specific process, the substrate can be heated to somedesired temperature during the deposition process. A feature of theinvention is that the film growth rate in the FGS is exponentiallydependent upon temperature, and it is therefore beneficial to keep onlythe substrate at a high temperature while chamber surfaces are kept at alow temperature for efficient, selective deposition. In the exampleembodiments the platen 44 is a PTFE or PFA coated thermally conductiveblock (e.g., formed of copper or aluminum) and is positioned along ornear an upper surface of the insulating substrate holder housing 43 toprovide uniform heating of the substrate while other portions of thehousing are maintained at low temperatures. The substrate holder housing43 may be made of PTFE or ceramic coated with PTFE or PFA or may be anactively cooled metal that is coated with, for example, PTFE or PFA.Such an active cooling arrangement can assure that surfaces of thesubstrate holder housing are kept at a relatively low temperature toprevent film formation thereover. At the same time, for example, thesubstrate may be heated by one or more heating elements 45 embedded inthe thermally conductive platen 44. The thermally conductive platen 44can be resistively heated by applying an electric current from an ACpower supply 46 to the heating elements 45. The substrate 33 is, inturn, heated by the platen. The conductive platen may be made of CVD-SiCor made of high conductivity metal like Cu or W—Cu alloy or Al coatedwith PFA or PTFE to protect the platen from chemical reaction withprocess solution. Although not illustrated, the heating elements used tocontrol the temperature of the film formation surface may alternativelybe formed in a horizontal orientation and may be in an array of parallelelements or a two dimensional matrix or mesh-like design to facilitateuniform generation and distribution of thermal energy and achieverelatively uniform temperature across the film-forming surface. Radiantheat generation is also contemplated, e.g., based on positioning ofradiant sources above or adjacent the film forming surface. For theillustrated embodiments, a temperature sensor 47, such as athermocouple, is also embedded in or near the platen to monitor thetemperature of the platen or substrate in a conventional manner. Themeasured temperature is used in a feedback loop to control the powersupplied to the heating element 45, whereby the substrate temperaturecan be maintained or controlled at a desired temperature suitable forthe particular process application.

The showerhead 41 which introduces a processing solution over thesubstrate surface 33 is located above the substrate holder assembly 31.The showerhead 41 receives processing solution and other liquids basedon the configuration of the liquid control panel, which directs thesupply of various liquids used in different steps of the processsequence or used in different steps of the chamber cleaning sequence.The showerhead allows processing solutions from the liquid control panelto be uniformly introduced and distributed over the substrate holderassembly. In the illustrated examples, the showerhead dimension issufficiently larger than the heated platen and the substrate to providea flow of cool processing solution along regions about the substrateholder housing adjacent the platen. Optionally a cooling channel may beformed inside the substrate holder housing to further limit temperatureelevation beyond the film forming surface of the substrate. In FIG. 4,an exemplary mechanical clamping unit comprising an upper clamp nut, alower clamp nut, and a clamp screw is also illustrated. The mechanicalclamp unit mechanically presses the spacer against the substrate. In anautomated system, the mechanical clamping unit may be operated by one ormore mechanical actuators.

FIG. 4 shows an embodiment in which the showerhead 41 is substantiallyfixed in position relative to the substrate holder assembly 31. It willbe appreciated that in other contemplated embodiments, it might bedesirable to use a moving, rotating, or oscillating showerhead asanother means of ensuring uniform flow of process liquid over the entiresubstrate surface. Skilled artisans can easily add such modificationswithout undue experimentation using electric motors, mechanical linkagesand the like as are well known in the art.

Embodiments of the reaction chamber 10 have been illustrated withexemplary fasteners for attaching components to one another and forattaching the workpiece within the reaction chamber for processing. Itwill be appreciated that a vacuum chuck may also be used to secure thesubstrate 33 to platen 44. Other designs are contemplated. The crosssectional view of FIG. 5 illustrates another exemplary embodiment of anFGS process chamber. In this embodiment, the substrate holder 31′incorporates a vacuum chuck including vacuum orifices 54 to securesubstrate 33. A ring 51 serves the function of spacer 35 (i.e., defininga volume of entrapped process liquid above the substrate. Substrateholder 31′ further incorporates a cooling jacket 52 maintained by theflow of coolant through inlet 53 and outlet 53′. The showerhead assembly41′ further incorporates a temporary solution reservoir 55, from whichprocess fluid is dispensed through capillary tubes 56 and microswitchvalves 57. Solution is added to temporary reservoir 55 through inlet 58.An outlet 59 provides pressure relief so that the pressure withinreservoir 55 is held constant.

It will be appreciated that the pattern of openings in the showerheadmay be relatively uniform across the face or it may be nonuniform. Forinstance, the showerhead may include two concentric zones: an inner zonecentered over the spacer seal for dispensing processing solution on tothe film growth surface of a substrate; and an outer zone for dispensingadditional processing solution to cool portions of the processingchamber outside the spacer opening which receives the substrate. Theinner showerhead zone primarily directs processing solution to the filmgrowth surface to effect chemical reactions which lead to film growth.Relatively hot, partially spent solution exits the spacer volume as newcool solution is dispensed therein. The outer showerhead zone primarilydirects cool solution to regions of the chamber beyond or outside thespacer opening. This flow of relatively cool solution serves to coolsides of the platen and portions of the substrate holder assembly whichmay be heated incidental to the heating of the substrate. The same flowcan mix with the relatively hot, partially spent solution that exits thespacer volume. The combined flow exits the chamber 10 via drain 34 mayreturn to solution reservoir 11 such as shown in FIG. 1. The returningsolution may be pre-cooled prior to entry into the reservoir 11.

It will be understood that flows through the inner and outer zones ofthe showerhead may be under separate control to optimize processparameters and quality of film growth. The inner zone of the showerheadmay receive chemical processing solution from a first inlet. The systemcontroller 20 directs the volume or volumetric flow rate and frequency(if not a continuous flow) for dispensing of solution into the spaceropening through the inner showerhead zone based on, for example,specific chamber geometries, a selected VSAR, the film forming chemistryand the reaction rate—all to achieve a satisfactory quality of filmformation. Flow through the inner zone is controlled and dispenses thesolution over,the spacer seal to react along the film growth surface ofa substrate. The outer zone of the showerhead may receive chemicalprocessing solution from a second inlet. The system controller directsthe volume or volumetric flow rate and frequency (if not a continuousflow) for dispensing of solution through the outer showerhead zone andinto regions of the chamber primarily outside the spacer opening basedon, for example, specific chamber geometries, heat transfercharacteristics of the components being cooled, temperature stability ofthe partially spent processing solution. Parameters may be optimized tominimize formation of undesirable precipitates in solution being cycledback to the reservoir and to minimize or eliminate formation of film on,for example, walls or other surfaces in the reaction chamber.

In one example of process conditions when the chamber is operated withthe dual zone shower head, the inner zone may be connected to a meteringpump which dispenses solution over the substrate opening every 20seconds in a volume which is one to two times the spacer volume. Thevolume dispensed may be a multiple of the spacer volume to quicklyquench the partially spent solution and assure complete replenishment ofprocessing solution in the spacer volume. At the same time, the outerzone may be programmed to dispense, with the assistance of a pump, acontinuous flow of 30 to 100 ml/min (for a chamber designed for coatinga 5×5 cm substrate) in regions outside of the spacer opening to cool thechamber surfaces and the solution which flows over the spacer.

Other arrangements which provide further optimization of processconditions include provision of processing fluid from the second inletat a lower temperature than processing fluid delivered from the firstinlet so that the outer zone of the showerhead provides solution withgreater cooling capacity while the inner zone provides solution whichrequires less heating in order to effect desired reactions along thefilm growth surface. That is, the solution can be cooled to atemperature which assures sufficient stability while being transferredand dispensed but which minimizes the amount of heat generated tosustain the growth surface at a minimum desired temperature. Forexample, the temperature of solution exiting the inner zone of theshower head may be in the range of 20-25° C. while the temperature ofsolution exiting the outer shower head may be in the range of 10 to 15°C. or lower. Optionally a cooling channel may be formed inside thesubstrate holder housing to further limit temperature elevation beyondthe film forming surface of the substrate. Thus it can be seen that theinventive apparatus and method provides the user with wide latitude toadjust the operating conditions for particular purposes.

In some applications, more than one layer of semiconductor material maybe formed on a substrate by a sequence of chemical processes. Referringto FIG. 6, there is shown a flow chart of exemplary steps thatsequentially place multiple layers of material on the substrate.Formation of two layers in a sequence may include a clean step forremoving contaminants from the substrate surface, followed by formationof a first layer. Next, a first post-film formation clean step isperformed for removing residual chemicals from the substrate surface,followed by a step for forming a second film layer different from thefirst layer. This is followed by a second post-film formation clean stepfor removing residual chemicals from the substrate surface. In thepre-film formation clean step, a substrate placed on the substrateholder assembly inside the chamber is cleaned with a first cleaningsolution, rinsed with de-ionized (DI) water, and dried, for example, bya spin drying method as is common in semiconductor device manufacturing.Next, a first semiconductor layer is formed along the surface of thesubstrate, followed by a postfilm formation clean wherein the substrateis cleaned with a second cleaning solution, rinsed with DI water for asuitable duration and dried. For multi-layer film formations, a systemcomprising a plurality of process chambers and a substrate handlingrobot can offer high operation efficiency and throughput resulting in arelatively low equipment cost per film layer. A single robot may providesubstrate handling for multiple processing chambers.

FIG. 7 depicts an exemplary multi-chamber system incorporating one ormore of the chambers illustrated in the foregoing figures and comprisinga substrate load/unload unit, an optional loadlock chamber, a pluralityof processing chambers, and a substrate handling robot. Each processingchamber may be dedicated to either a reaction or a clean. In theexemplary system of FIG. 7, the processing chambers include a firstclean chamber, a first reaction chamber, a second reaction chamber, athird reaction chamber, and a second clean chamber. In operation of themulti-chamber system of FIG. 7, a substrate cassette loaded with one ormore substrates is placed in the substrate load/unload unit. Thesubstrate handling robot removes a substrate from the load/unload unitand places it in a selected process chamber under direction of a systemcontroller (not shown). After deposition is complete, the robot picks upthe substrate from the processing chamber and transfers the substrate toanother chamber or to the substrate cassette in the substrateload/unload unit. In other embodiments, the substrate load/unload unitmay handle a plurality of substrate cassettes for higher operationalefficiency or may simultaneously process wafers in different chambers.To provide high pressure chemical film growth conditions in the systemof FIG. 7, the loadlock chamber may be configured to include or movablyreceive the substrate storage cassette. With such an arrangement, highersystem operating efficiencies can result as the frequency of pressureequalization for substrate transfer between the substrate load/unloadunit and the individual chambers is reduced. In other embodiments, somechambers or all chambers may be equipped to perform both chemicalprocessing and cleaning.

FIG. 8 depicts another exemplary multi-chamber FGS according to theinvention.

The multi-chamber system comprises a substrate load unit, a loadchamber, a substrate unload unit, a plurality of processing chambers,and a substrate handling robot positioned to move substrates betweenunits and one or more chambers. The processing chambers include a firstclean chamber, a first reaction chamber, a second reaction chamber, athird reaction chamber, a fourth reaction chamber, and a second cleanchamber, each chamber being dedicated to either chemical film growth ora cleaning operation. A substrate cassette loaded with one or moresubstrates is placed in the substrate loader. The substrate handlingrobot in the load chamber picks up a substrate and places in a selectedprocess chamber by a system controller (not shown). After deposition iscomplete, the robot picks up the substrate from the processing chamberand transfers the substrate to the substrate cassette in the substrateunload unit. In other embodiments, the substrate load unit and unloadunit may handle a plurality of substrate cassettes for higheroperational efficiency.

It will be appreciated that in some instances it is desirable to deposita film onto a somewhat continuous, flexible substrate 33′ in aroll-to-roll configuration. The inventive apparatus may be modified asshown schematically in FIG. 9 by modifying the spacer assembly 35′ toform a thin framework that can be lowered against a selected portion ofthe substrate 33′ to secure the substrate against the heater block 44and define the volume of solution held over the area of deposition 49.Any conventional means may be used to provide a fluid seal when thespacer assembly 35′ contacts the film, such as gaskets, O-rings orvarious compliant materials. The substrate may be handled by anyconventional roll-to-roll handler, which may preferably includetensioning rolls and drive mechanisms on either the feed roll or thetake-up reel, or both. Vacuum delivered via one or more orifices 54 maybe used to hold substrate 33′ flat against heater block 44. Insulation61 may be placed between heater block 44 and cooling jacket 52.

The operation of the system illustrated in FIG. 9 is similar to that ofother embodiments, with the following differences: First, the flexiblesubstrate 33′ is fed from the feed roll and secured to the take-up roll.Second, the spacer assembly 35′ is lowered against the substrate,securing substrate 33′ against the heater block and defining the areaupon which film deposition will occur. Deposition is carried out in thesame manner as in the previously-described embodiments. When a film ofadequate thickness has been deposited, spacer assembly 35′ is raised andthe substrate is indexed forward to bring a new area under the spacerassembly for the next deposition.

It will be appreciated that downstream operations such as washing,drying, heat treatment, dicing, etc., may be carried out in any desiredorder and may be done while the substrate is still continuous or may bedone after individual “panes” have been diced from the roll. Heattreatment, in particular, may be carried out by passing the continuousfilm through an oven, microwave cavity, or radiant heater zone, with thefinal take-up roll located at the opposite end of the heating apparatus.Conventional means may be employed to buffer the movement of the filmbetween the deposition stage and the heat treatment stage if desired.Rinsing and drying may be carried out in the buffer zone, if desired, sothat the film may enter and move through the heat treatment stage in auniform manner without excess moisture.

The exemplary film growth systems described in FIGS. 1-8 may be used toform films comprising any of a wide range of materials, includingmetals, semiconductors, and insulators on a temperature-controlledsubstrate from constituents in a solution by series of reactions whichare performed while controlling one or more of: the substratetemperature, the chamber pressure, the flow rate of the processingsolution, the pH and the composition of the solution. The followingdescription of a chemical process suitable for use in one of theaforedescribed chambers will lead to formation of an intrinsic II-VIcompound semiconductor film. In this example, a set of chemicalreactions provide relatively uniform and low defect formation of astoichiometric II-VI compound film, e.g., CdS. This is to be comparedwith the film quality achievable via physical vapor deposition that mayinvolve sputtering of the Group II and VI species. Generally, a mix ofone or several ligands is combined in an aqueous solution containing agroup II salt having an anion that can act as a supplementary andsomewhat competitive complexing agent of the group II element cation ofthe salt to form a complex of low or moderate stability. The solutionfurther includes an OH⁻ source for pH control, and a source of a groupVI element, all mixed together in the right proportion in an aqueousbath to grow an intrinsic II-VI compound film on a substrate. Thesubstrate forms the base of the replenishable shallow solution bath. Thesubstrate is preferably hydrophyllic, thereby facilitating accumulationof OH⁻ bonds along a surface thereof. If the OH″ is derived from NH₄OH,then an ammonium salt buffer, NH₄Cl, with the same anion as that of thegroup II salt, CdCl₂, is also included in the solution to avoiddepleting OH⁻ and NH₃. When this solution is maintained at 5 to 15° C.there will be substantially no reaction between the OH⁻ radicals and thecomplexes for more than two weeks (very good shelf life). When thedeposition is performed in the chamber on a 5 cm×5 cm substrate(nominal), the aqueous solution may be formed in the followingproportions:

CdCl2 0.0015M Ammonium chloride (NH₄Cl)  0.006M Ammonium hydroxide(NH₄OH)  0.77M Potassium nitrilotriacetate (NTA or N(CH₂OOK)₃) 0.0009MThiourea ((CS(NH₂)₂)  0.003M Ligands: Cl−, NH₃ (from NH₄OH), NTA(N(CH₂OOK)₃) Group II salt: CdCl₂ Group VI element source: Thiourea(CS(NH₂)₂) OH− source: NH₄OH NH₃ buffer: NH₄Cl

The constituents dissociate as follows:

NH₄Cl→NH₄+Cl⁻  1)

NH₄OH→NH₄+OH wherein NH₄ ⁺+OH⁻→NH₃+H₂O   2)

KOH→K⁺+OH⁻  3)

CdCl₂→Cd²⁺+2Cl⁻  4)

The NH₄Cl and NH₄OH regulate the OH⁻ concentration in the solution. Withsufficient OH⁻ concentration in the solution, there will be asignificant number of bonds between the OH⁻ radicals and sites along thehydrophilic substrate so as to facilitate reaction of complexescontaining the Group II element, e.g., Cd, to form an intermediaryCd—hydroxyl compound bound to the substrate with the help of heat.

With dissociation of CdCl₂ per (4), above, in the presence of the NH₃ligand, cadmium ions form first Group II based complexes (I) accordingto:

Cd²⁺+nNH₃→[Cd(NH₃)_(n)]²⁺  (I)

where n=1, 2, 3, or 4. The stability of this complex depends on thevalue of n. The higher the value of n, the more stable the complexbecomes. The potassium nitrilotriacetate, N(CH₂OOK)₃, or NTA also actsas a ligand forming second Group II based complexes (II) according to:

Cd²⁺+nN(CH₂OOK)₃→[Cd(N(CH₂OOK)₃)_(n)]²⁺  (II)

The Cl⁺ supplied by CdCl₂ and NH₄Cl acts as a third ligand to form athird Group II based complex according to:

Cd²⁺+nCl⁻→[Cd(Cl)_(n)]^((2+)+(n−))   (III)

The use of multiple ligands can optimize binding of Cd²⁺ or othermetallic ions of interest into complexes which stay in solution andpermit better control of the growth of high quality intrinsic films,allowing for on-demand delivery of the metallic ion at the reactionsite:

OH⁻[substrate]+[Cd(L)_(n)]^(m)→^(heat)Cd(OH)⁺[substrate]+nL^(p)

wherein L represents any of one or more suitable ligands, e.g., NH₃,N(CH₂OOK)₃, or Cl⁻. and [Cd(L)_(n)]^(m) represents one of thecorresponding complexes containing Cd²⁺.

Furthermore, to complete the formation of CdS principally at thesubstrate surface, the thiourea ((NH₂)CS) in the presence of asufficient OH⁻ concentration first undergoes a partial reaction as shownbelow to produce HS⁻:

(NH2)CS+OH⁻→SH⁻+H₂O+H₂CN₂

With the metastable Cd(OH)⁺ already bound to the substrate and with HSpresent in surrounding solution, it becomes possible to convert all ofthe Cd(OH)⁺ to CdS on the heated substrate:

Cd(OH)⁺[substrate]+SH⁻→^(heat)CdS[substrate]+H₂O.

Since the Cd²⁺ ions are optimally and primarily bound in a hydroxylmetastable compound at the desired site of deposition, the spontaneousformation of Cd(OH)₂ that will eventually lead to the precipitation ofCdS in the solution tank is inhibited. To further ensure prevention ofprecipitate which would then deposit out of the solution, the solutionreservoir containing the aqueous solution which flows into the chamberis maintained at low temperature, e.g. 5 to 15° C. That is, generallythere is inhibition of the reaction

Cd²⁺+OH⁻→Cd(OH)₂

which would typically be present in a chemical bath deposition techniquethat attempts to deposit poor quality CdS directly from the aqueoussolution on to a substrate, e.g.,

Cd²⁺+S²⁻→CdS.

Instead of depositing CdS as a precipitate in a bath and on a substratesimultaneously, it is possible to create the CdS on a hydrophilicsubstrate surface only. Further, as a substrate surface becomescompletely covered with CdS, the reaction continues as hydroxyl ionspresent in the aqueous solution associate with the newly deposited CdSto continue formation of the Cd(OH)⁺ bound to the exposed CdS. With HS⁻present in surrounding solution, it becomes possible to convert all ofthe Cd(OH)⁺ to CdS:

Cd(OH)+[CdS]+SH⁻→^(heat)CdS[CdS]+H₂O.

Generally, there is little or no precipitation or true deposition of theCdS on the substrate. Instead, the CdS is formed along the growthsurface in a replacement reaction. Further, with the Group II elementbound in complexes while in solution, there is little or no opportunityfor formation of undesirable precipitates of the metallic ion, e.g.,Cd(OH)₂ when forming CdS. A feature of the invention is the simultaneousprovision of a series of reactions that prevent deposition of the GroupII element containing compound by precipitation and allow formation ofthe Group II element containing intermediary compound bound along thesubstrate surface prior to formation of the II-VI compound.

To facilitate selective growth of the II VI film by the aforedescribedreplacement technique, the following conditions are believed to beuseful:

-   a. The substrate is maintained at a temperature ranging between 100    to 200° C., depending on the substrate, preferably 170° C. for a    glass substrate.-   b. The solution is periodically replenished over the growth area    with an injection which washes away solution which has been present.    The interval time for injecting fresh solution over a substrate    surface in the illustrated chamber may range from 10 to 30 seconds,    with 20 seconds having been preferred in some experiments.-   c. The volume of the injected solution may be one to two times the    volume of the solution being replaced e.g., for the 5×5 cm substrate    having an overlying solution volume of about 3 ml and the    replenishing solution is at least 3 ml uniformly dispensed into the    bath to displace the spent solution.-   d. The reactions which might otherwise form precipitates and lead to    deposition on surfaces other than that of the substrate are limited    by cooling the outer part of the heater assembly and the washed    solution, e.g., the outer part of the heater assembly outside the    reaction bath area may be continuously or intermittently cooled with    the relatively cold solution flowing from the reservoir. As this    solution accumulates at the bottom of the chamber, it is pumped back    into the solution reservoir for further cooling and recirculation.    For the 5×5 cm substrate in the film growth system, the flow rate of    this solution may be about 120 ml/min.

The exemplary process described above was specifically directed to thegrowth of CdS films. It will be appreciated that many other useful filmcompositions can benefit from the inventive apparatus and method. Thefollowing table presents the reagent formulations that Applicant hasfound suitable for creating films of InOSe, InOS, CuS, MnO, LiMnO, CdO,ZnO, CdS, Cu, CdAlSSe, ZnS, and CdAlS. In each case it can be seen thatmultiple ligands are combined in order to achieve a bath solution thatexhibits a desired combination of stability when cool with ease ofreaction when warmed adjacent to the substrate. This combination cannotbe achieved with a single ligand or “complexing agent” as taught, forexample, by McCandless et al '845.

Concentration Remarks/Comment InOSe InCl₃ 0.002M Acetic acid  0.04MLigand Selenourea  0.08M Ligand and source of Se Hydrogen peroxide  2.4MInOS InCl₃ 0.002M Acetic acid  0.04M Ligand Thiourea  0.08M Ligand andsource of S Hydrogen peroxide  2.4M CuS CuSO₄ 0.005M Triethanolamine 0.09M Ligand Citric Acid 0.125M Ligand Potassium nitrilotriacetate 0.03M Ligand Thioacetamide 0.008M Ligand and source of S MnO MnCl₂0.002M Sodium citrate 0.008M Ligand Potassium nitrilotriacetate  0.01MLigand Ammonium hydroxide  0.6M Ligand and source of O LiMnO MnCl₂0.0016M  LiCl 00004M  Sodium citrate 0.008M Ligand Potassiumnitrilotriacetate  0.01M Ligand Ammonium hydroxide  0.6M Ligand andsource of O CdO CdCl₂ 0.002M Sodium citrate 0.0056M  Ligand Ammoniumhydroxide  0.77M Ligand Potassium nitrilotriacetate 0.0009M  LigandHydrogen peroxide  3.6M ZnO Zn(NO₃)₂ 0.0036M  Sodium citrate 0.006MLigand Ammonium hydroxide  0.77M Ligand Potassium nitrilotriacetate0.0009M  Ligand Hydrogen peroxide  3.6M CdS CdCl₂ 0.0015M  source of Cdand Cl (another ligand) Ammonium chloride 0.006M Ligand buffer Ammoniumhydroxide  0.77M Ligand Potassium nitrilotriacetate 0.0009M  LigandThiourea 0.003M Cu CuSO₄•5H₂O 0.0096M  Hydantoin 0.107M Ligand CitricAcid 0.004M Ligand Potassium nitrilotriacetate  .05M Ligand Hydrazine 1.6M Potassium hydroxide  0.22M CdAlSSe CdCl₂ 0.001M Al(NO₃)₃ 0.001MSodium citrate 0.008M Ligand Ammonium hydroxide  0.6M Ligand Potassiumnitrilotriacetate 0.0009M  Ligand Thiourea 0.004M Sodium selenosulfite0.004M ZnS Zinc Acetate 0.004M source of Zn and acetate (another Ligand)Sodium citrate 0.0008M  Ligand Thioacetamide 0.004M Ligand and source ofS CdAlS CdCl₂ 0.001M Al(NO₃)₃ 0.001M Sodium citrate 0.008M LigandAmmonium hydroxide  0.6M Ligand Potassium nitrilotriacetate 0.0009M Ligand Thiourea 0.004M

It will be appreciated that the invention is not limited to theexemplary formulations and combinations of reagents presented in theforegoing table. Skilled artisans will notice that there is wideflexibility in the choice of soluble metal salts with examples ofchlorides, nitrates, citrates, acetates, and sulfates having beendemonstrated. Similarly, there is a wide choice of ligands includingacetic acid, triethanolamine, citric acid, potassium nitrilotriacetate,sodium citrate, ammonium hydroxide, ammonium chloride, and hydantoin.Thus, the user may select particular soluble species and suitableligands for a particular application through routine experimentation.

For some applications, the films deposited by the chemical methodoutlined above are typically subject to a heat treatment or annealingstep to develop the desired microstructure, crystallinity, and otherproperties for particular applications. Those skilled in the art willappreciate that the preferred heat treatment will depend on severalfactors such as the compound being formed and the type of substrate. Thefollowing table presents some typical heat treatments, and the skilledartisan can easily adapt these results to other systems with routineexperimentation.

Exemplary heat treatments in the tube furnace:

Coating Substrate Anneal T (° C.) Time (min) Atmosphere CdS glass 300 60Ar ZnO glass 400 60 Ar CdAlS glass 275 60 Ar CdO glass 400 60 Ar

It will be appreciated that heat treatment may be carried out using anysuitable means, including but not limited to convective, conductive,radiative (including flash lamp, laser, and/or IR heating) and microwaveor RF heating. In some cases, microwave heating may be faster thanconvective or conductive heating, for example. Radiative methods may beused when it is desirable to apply heat quickly to the film whileminimizing heating of the substrate, for instance when the substrate isa polymeric material. Conductive heating may be desirable when thesubstrate is metallic and is therefore a good thermal conductor. Throughroutine experimentation the skilled artisan can therefore select theoptimal heat treatment method for a particular combination of film andsubstrate.

Materials made by the inventive process display several improvements inuseful properties. One accepted method for assessing the adherence of afilm is the simple adhesive-tape test. This test was performed on theCdS films deposited on glass by the traditional CBD and the onedeposited by the current invention with no post deposition annealing.After three repetitions, the film deposited by the traditional CBD wassuccessfully removed, whereas the film grown by the current inventionstill remained intact after twenty repetitions. The CdS film also showedsuperior optical properties compared to films grown by traditional CBD.

Chemical Recycling

Spent solution can be reused in the coating process by replenishing theaqueous solution with depleted reactants. It will be appreciated thatafter several cycles of replacing depleted reactants, there may be ahigh build-up of undesirable by-products, rendering the solutionunsuitable for high quality film growth. As a result, the remainingquantity of Group II metal, e.g., Cd, could be precipitated out of thesolution in the form of CdS by adding excess KOH, (NH₂)CS to thesolution and heating the solution to about 90° C. The CdS precipitatecan then be filtered out of the mixture and washed. Next, the CdSprecipitate can be dissolved in HCl according to the equation

CdS+HCl→CdCl₂+H₂S⇑

The process is performed under a ventilation hood in the presence of acarbon filter which will absorb essentially all of the H₂S gas. TheCdCl₂ solution may then be purified, recrystallized, and dried forreuse.

Doping of Compound Semiconductor Material During Film Growth

In lieu of forming intrinsic CdS as described above, e.g., creating theCdS on a hydrophilic substrate surface, the intrinsic film can be dopedin-situ by replacing less than 1% concentration of the matrix salt withthe appropriate dopant salt. For n-type doping the dopant salt cationneeds to be a group III ion and for p-type doping the dopant salt cationneeds to be a group I ion. Alternately, ex-situ doping of the intrinsicfilm can be achieved by ion-implantation of group III ions for n-typeconductivity and group V ions, preferably nitrogen ions, for p-typedoping. Ex-situ doping can also be achieved in a FGS by flowing a lowconcentration of a dopant solution on the substrate with the intrinsicfilm subjected to a temperature ranging between 80 to 250° C. Thefollowing salts may be used to provide dopants:

-   Group III salt of interest: AlCl₃, GaSO₄, InCl₃, etc.-   Group I salt of interest: AgNO₃, KNO₃, LiNO₃.

The invention may be used to form multiple layers of material includingp-n semiconductor junctions. A second exemplary film that may be formedover CdS is CdSe. An exemplary processing solution for such filmformation is as follows:

CdCl₂ 0.0023M  Ammonium chloride (NH₄Cl) 0.005M Ammonium hydroxide(NH₄OH)  0.60M Potassium nitrilotriacetate (NTA or N(CH₂OOK)₃) 0.0009M Na₂SeSO₃ 0.007M Na₂SO₃ 0.004M

It will be understood by those familiar with chemical processing that innumerous applications such as manufacture of solar cells numerous otherlayers may be deposited on a substrate and a variety of cleans will beperformed as intermediate and pre- and post-processing steps. It will befurther understood that when more than one material is deposited on asubstrate, the individual material layers may be deposited in anydesired sequence to create a multilayer device having a desiredfunctionality.

Deposition of Intrinsic ZnO Films

Zinc oxide films were grown according to the recipe shown in the tableof formulations. As grown, the film was amorphous and highly insulative.After annealing for 1 hour at 400° C. in Ar, some reduction inresistivity was observed. However, Applicant discovered, surprisingly,that the resistivity of the inventive material is higher, and thecorresponding carrier concentration is lower, than what has beenreported for undoped ZnO films grown by prior methods, as summarized inthe following table:

Post 400 to 1000 nm Undoped ZnO deposition Carrier average Carrier conc.Resistivity Deposition method Annealing type Substrate Thicknesstransmission (cm⁻³) (Ω-cm) This invention Yes n Borofloat glass 0.4 μm95% 3.54 × 10¹⁴ 616 Radical-source MBE^(a) No n Glass and 0.49 μm  90% 1.5 × 10¹⁸ 0.09 Sapphire Pulsed Laser Yes n Sapphire 0.4 um NR 4.59 ×10¹⁵ 0.48 Deposition^(b) Metalorganic Vapor No n Sapphire NR NR 1.58 ×10¹⁶ to 0.3 to 565 Phase Epitaxy^(c) 6.60 × 10¹⁷ Pulsed Laser No nSapphire NR NR   2 × 10¹⁸ 0.09 Deposition^(d) DC reactive No NR^(f)Glass NR 75% NR NR sputtering^(e) ^(a)R. Hunger et al., Mat. Res. Soc.Symp. Proc. Vol. 668 (2001) ^(b)T. Oshima et al., Thin Solid Films 435:49-55 (2003) ^(c)Y. Ma et al., Journal of Crystal Growth 255: 303-07(2003) ^(d)Y. Ryu et al., Appl. Phys. Letters 83[1]: 87 (2003) ^(e)A.Mosbah et al., Surface and Coating Technology 200: 293-96 (2005) ^(f)NR= not reported

The inventive ZnO film as grown is amorphous and has a band gap muchhigher than that of bulk ZnO as shown in FIG. 10; although the exactmechanism for this is not known with certainty, Applicant speculatesthat it might be due to quantum shift caused primarily by thenano-particles constituting the film or to the presence of ZnO₂ phase.However, after annealing the film recrystallizes, resulting in band gapof about 3.2 eV, which is similar to that of bulk ZnO. The x-raydiffraction pattern of the annealed film shown in FIG.11 clearlyindicates that the film is the hexagonal ZnO phase with an estimatedcrystallite size of 22 nm.

One can also see from the table and FIG. 10 that the opticaltransmission of the inventive film is also superior to those of priorart ZnO films. This shows that the inventive material will be suitablefor many applications where good optical transmission is required.

The properties of the inventive material make it extremely well suitedfor carrier type engineering. The p- or n-type doping could be done insitu or ex situ followed by annealing to recrystallize the film andallow the dopant to naturally occupy the correct lattice position andbecome activated. Some exemplary dopants and their source compoundsinclude the following:

-   a) For in-situ n-type doping, preferred sources are: In salt    (In(NO₃)₃ or InCl₃), Ga salt (Ga(NO₃)₃), or Al salt (Al(NO₃)₃ or    Al₂(SO₄)₃), or other group III element containing salts.-   b) For in-situ p-type doping preferred sources are: Li salt    (Li(NO₃), or Ag salt (Ag(NO₃), or group I or IV element containing    salts.-   c) For ex-situ n-type doping: a very thin layer of In₂O₃, or Ga₂O₃,    or Al₂O₃ may be deposited on the as grown ZnO film before annealing.    Alternatively, ion implantation of In, or Ga, or Al may be used.-   d) For ex-situ p-type doping: N ion implantation would be a    preferred approach.

All the foregoing doping processes are preferably followed by annealingat temperatures ranging from 300 to 1000° C., depending on the choice ofsubstrate, in Ar, N₂, H₂, O₂, air, or some combination of these gases ina suitable tube or muffle furnace or in a rapid thermal annealingsystem.

The table below show some preliminary Hall measurement data of thein-situ n-type doped inventive ZnO using In(NO₃)₃ and Ga(NO₃)₃ salts.The films were annealed at 400° C. in Ar for one hour before the Hallmeasurement.

Zn-salt to carrier dopant-salt carrier concentration ResistivityMobility Sample ID dopant ratio type (cm⁻³) (Ohm-cm) (cm²/Vs) ZnO: Al80207-2 Al 100:1 n 9.83 × 10¹⁵ 466 1.36 ZnO: In 80207-3 In  9:1 n 1.31 ×10¹⁷ 23.3 2.05

The inventive ZnO film may be deposited on a variety of substrates andcombined with other familiar structures such as electrodes, etc., toform various useful devices. Some examples include the following:transparent conducting oxide for current collection in solar cells,light emitting diodes, and flat panel displays; ZnO based light emittingdiodes; matrix material for soft magnets used in spintronics; sensorsand detectors. It will be appreciated that the substrate may be rigid(e.g., glass or ceramic) or flexible (e.g., sheet metal or polymerfilm); it may also include one or more electrodes. Electrodes may beprovided as a separate metallic coating on a substantially insulatingsubstrate (e.g., Cu on polyimide); alternatively, the substrate itselfmay be used as an electrode in the case of a sheet metal substrate.Electrodes may be substantially continuous over the area occupied by thecoating or they may be discontinuous or arranged in any selected pattern(e.g., in order to define selected active areas, arrays of pixels,etc.).

Although numerous embodiments of the invention have been illustrated anddescribed, the invention is not so limited. Numerous modifications,variations, substitutions and equivalents will occur to those skilled inthe art without departing from the spirit and scope of the presentinvention.

1. A method for depositing a solid ZnO film onto a substrate from areagent solution comprising the steps of: providing a supply of reagentsolution containing a source of Zn, a source of O, and at least twoligands maintained at a first temperature at which homogeneous reactionsare substantially inhibited within said reagent solution; dispensing acontrolled flow of said reagent solution from a showerhead assembly;positioning said substrate to receive at least a portion of saidcontrolled flow of said reagent over a selected area of said substrate;providing a raised structure peripheral to said selected area whereby acontrolled volume of said reagent solution may be maintained upon saidsubstrate and replenished at a selected rate; and, heating saidsubstrate and said controlled volume of said reagent solution upon saidsubstrate to a second temperature, higher than said first temperature,whereby deposition of zinc oxide from said reagent solution may beinitiated.
 2. The method of claim 1 wherein said second temperature isat least 60° C. higher than said first temperature.
 3. The method ofclaim 1 wherein said source of Zn comprises a soluble Zn salt.
 4. Themethod of claim 3 wherein said soluble Zn salt is selected from thegroup consisting of: chlorides; nitrates; citrates; sulfates; andacetates.
 5. The method of claim 1 wherein said at least two ligands areselected from the group consisting of: acetic acid; triethanolamine;citric acid; potassium nitrilotriacetate; sodium citrate; ammoniumhydroxide; ammonium chloride; and hydantoin.
 6. The method of claim 5wherein said at least two ligands comprise sodium citrate, ammoniumhydroxide, and potassium nitrilotriacetate.
 7. The method of claim 1further comprising the step of: annealing said coated substrate at aselected temperature in a selected atmosphere.
 8. The method of claim 7wherein said selected temperature is in the range of 300 to 1000° C. andsaid selected atmosphere is selected from the group consisting of: Ar;N₂; H₂; O₂; air; and combinations thereof.
 9. The method of claim 1wherein said reagent solution further comprises a soluble salt of adopant species having a valence different from that of Zn(II).
 10. Themethod of claim 9 wherein said soluble salt is selected from the groupconsisting of: In(NO₃)₃; InCl₃; Ga(NO₃)₃; Al(NO₃)₃; Al₂(SO₄)₃; salts ofother group III elements; LiNO₃; AgNO₃; salts of other group I elements;and salts of other group IV elements.
 11. The method of claim 1 furthercomprising the step of: treating said ZnO film by ion implantation of aspecies selected from the group consisting of: In; Ga; Al; and N.